methylphenidate meth·yl·phen·i·date (měth'əl-fěn'ĭ-dāt', -fē'nĭ-)n. A drug chemically related to amphetamine and that acts as a mild stimulant of the central nervous system, used especially in the form of its hydrochloride for the treatment of narcolepsy in adults and hyperkinetic disorders in children.